Polyergic cyclotron

ABSTRACT

A device for the production of a multiple energy proton beam comprising a negative hydrogen ion cyclotron with stripping foils placed at different radial and azimuthal positions. The negatively charged hydrogen ions at the various energy levels represented by the radial positions of the foils are stripped of their electrons thereby becoming protons which reverse their direction of curvature because of the reversed polarity and exit from the cyclotron along paths that, with proper azimuthal positioning of the foils, meet at a single point outside the cyclotron. A circular combining magnet centered on this point bends the proton trajectories as required to continue along a common path thus creating the multiple energy or polyergic beam.

United States Patent [151 3,641,446

Gordon 5] Feb. 8, 1972 [54] POLYERGIC CYCLOTRON Primary Examiner-RaymondF. Hossfeld [72] Inventor. Hayden S. Gordon, Berkeley, Calif. AtmmeyHa"y A. Herbert, JL and Robert K Duncan [73] Assignee: The United Statesof America as represented by the Secretary of the Air BSTRACT A devicefor the production of a multiple energy proton beam [22] Filed; I); 18,1969 comprising a negative hydrogen ion cyclotron with stripping foilsplaced at different radial and azimuthal positions. The

[21] App!" 886,237 negatively charged hydrogen ions at the variousenergy levels represented by the radia! positions of the foils arestripped of 52 us. CL ..328/228 313/62 328/234 their e'ecmns herebybewming WhiCh reverse [5]] Int. Cl H6511 13/00 direction of curvaturebecause of the reversed polarity and [58 1 mm olSearch ..328/228, 229,230, 234; mm the Paths 313/62 azimuthal positioning of the foils, meetat a single point outside the cyclotron. A circular combining magnetcentered on I 56] References Cited this point bends the protontrajectories as required to continue along a common path thus creatingthe multiple energy or UNITED STATES PATENTS p y s beam- 2,872,5742/1959 McMillan et al ..328/234 3 Claims, 6 Drawing Figures VICIVJN TalkPOLYERGIC CYCLOTRON BACKGROUND OF THE INVENTION 1. Field of theInvention Energetic particle generators, particularly proton generatingcyclotrons.

2. Description of the Prior Art Facilities for simulating thehigh-energy penetrating radiations of space require apparatus forgenerating beams of protons, one of the principal constituents of spaceradiations. Since proton radiation in space has a wide energy range andis for the most part isotropic, the proton beams generated for the testfacility should be polyergic and should irradiate the sample fromdifferent directions in order to simulate space radiations as closely aspracticable. Present proton beam generators produce monoergic beamsmaking it necessary to provide a separate generator for each energylevel represented in the simulating radiation. This greatly increasesthe cost and complexity of the test facility.

SUMMARY OF THE INVENTION The purpose of the invention is to provide asingle apparatus capable of producing a polyergic beam of protons.Briefly, the apparatus comprises a negative hydrogen ion (H') cyclotronin which a plurality of stripping foils are placed at different radialand azimuthal positions. The H ions at various energy levelscorresponding to the radial positions of the foils are stripped of theirelectrons by the foils, leaving hydrogen nuclei or protons which, beingpositively charged, follow a path of reversed curvature out of thecyclotron. With proper azimuthal positioning of the stripping foils thetrajectories of the exiting protons at the various energy levels may bemade to converge on and intersect at a point outside the cyclotron. Acombining magnet of constant field strength located at this point isdesigned to turn each of the component proton trajectories through theproper angle to have all trajectories continue along a common path, thusforming a polyergic beam.

As a further feature of the invention, the cyclotron may be designedwith the dee structure, or particle accelerating electrode structureoccupying roughly one-third of the main magnet space and with a set ofstripping foils in each of the two remaining thirds for producing, withthe aid of external combining magnets, a pair of polyergic beams in themanner described above. In providing a radiation test facility, it isdesirable that the test chamber contain a number of virtual radiationsources at different locations for radiating the test sample fromdifferent directions in order to simulate the isotropic nature of spaceradiation. The use of such dual-beam cyclotrons to feed the virtualsources (through suitable beam transport systems) reduces the polyergiccyclotron requirement to half the number of virtual sources.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic sectional viewtaken along the median plane of a polyergic cyclotron in accordance withthe invention;

FIG. 2 is a diagram showing the geometrical relationships at thecombining magnet;

FIG. 3 is a graph relating the product of combining magnet flux densityand radius required to produce a collimated beam to the turning anglesat the various energy levels and to the angular separation of theincident proton beams of highest and lowest energy level;

FIG. 4 is a large-scale illustration of a stripping foil in the form ofa paddle inserted into the ion beam envelope; and

FIGS. 5a and 5b are plan and elevation views respectively of a strippingfoil in the form of a rotatable ribbon extending across a portion of theion beam envelope.

DESCRIPTION OF THE PREFERRED EMBODIMENT In FIG. 1, which is a schematicsectional view taken along the median plane of a single dee, azimuthallyvarying field all (AVF) cyclotron, l is a pole face of the main magnethaving the spiral hills and valleys characteristic of the most advancedtype of AVF cyclotron and 2 is the outline of the main magnet coil. Theconstruction shown provides for the generation of two polyergic beams asalready mentioned and as will be explained more fully later. Inthis'design the dee structure, or particle-accelerating electrodestructure, the outline of which is shown at 3, is restricted to roughlyone-third of the angular space of the cyclotron in orderto providesufficient angular space outside the dee structure for the generationand extraction of polyergic protons in accordance with the invention. Aswell understood, charged particles, in this case negative hydrogen ions,are introduced at the center of the cyclotron and follow generallycircular paths about the center that increase in radius due to theirgain in kinetic energy with each passage through the dee structure. Theorbits of particles at several energy levels ranging from 20 mev. tomev. are labeled in FIG. I. Cyclotrons, including the AVF type, are wellknown and adequately covered in the literature, for example, inPrinciples of Cyclic Particle Accelerators by Livingood, D. Van NostrandCo., Inc., 1961.

The invention is not in any way limited to use with AVF cyclotrons, butmay be applied to any negative hydrogen ion accelerator in which thecharged particles orbit with increasing radius as the particle velocityincreases. This includes fixed-frequency cyclotrons, usually referred tosimply as cyclotrons, and synchrocyclotrons. The most likely applicationis to cyclotrons because of their high duty factor compared tosynchrocyclotrons. The invention may be applied to any type of cyclotronfrom the simple type with smooth magnet pole faces to the most advancedtype having magnet faces with spiral hills and valleys to which theinvention is shown applied in the drawing. The choice depends upon themaximum particle energy desired. The maximum energy obtainable in simplecyclotrons with smooth pole faces is limited by the relativisticincrease in particle mass as the particle velocity increases. Since theorbital period t is directly related the particle mass in accordancewith the equation where M is the particle mass, B is the magnetic fieldstrength, and q is the charge on the particle, the increasing massincreases the period t until the particle becomes so far out of phasewith the accelerating radio frequency field at the dee that no furtheracceleration occurs. To counter this effect B may be increased withincreasing radius in such manner as to keep the ratio M/B constant. Theincrease in B may be accomplished by a radical decrease in the magnetair gap; however, the resulting bowing of the magnetic lines in the gapproduces forces on the particles tending to drive them away from themedian plane of the cyclotron or, in other words, tending to destroy theaxial focus. This defocusing effect can be counteracted by alternatelyincreasing and decreasing the air gap azimuthally, through providingalternate high and low sectors in the pole faces, in order to produce analternating gradient of the field strength in azimuth. This produces netforces on the particles tending to hold them in the median plane andtherefore to preserve the axial focus. This effect is enhanced if theboundaries of the high and low pole face sectors are spiral. Therefore,by the use of alternate spiral high and low sectors in the pole faces aradial decrease in air gap can be employed to counteract the effect ofrelativistic particle mass increase without destroying the axial focusof the cyclotron. This construction, including the spiral boundaries, iswell known in the art and describe in the literature, for example, inLivingood cited above. U.S. Pat. No. 2,872,574 to McMillan et al. isanother example of the use of an azimuthally varying field, but withoutthe spiral modification, to offset the axial defocusing effect of aradially decreasing airgap.

The construction details of cyclotrons in general are well known in theart and are adequately described in the literature, Livingood and thepatent to McMillan et al. again being cited as examples. Negativehydrogen ions, which are hydrogen atoms with one electron attached, maybe introduced near the center of the cyclotron by a probe insertedthrough an axial passage in one of the magnet poles as shown in FIG. 6of the McMillan et al. patent. The ion generator and the specific designof the probe are not a part of the invention, any suitable designcapable of constantly delivering ions to the entrance gap of the deestructure near the center of the cyclotron is sufficient.

The only modification of a conventional cyclotron necessary for theaddition of the invention is possibly a restriction of the angularextent of the dee structure in order to provide room for the insertionof the stripping foils and for the passage of the generated protonstreams out of the cyclotron, as will be described later. The deestructure 3 employed in FIG. 1 is of the single-dee type. Dee structuresin general including the single-dee structure are described inLivingood. Early cyclotrons employed a dee structure having two deeseach of approximately 180 extent. The radiofrequency energization of thetwo dees was such that their voltages were of equal amplitude andopposite phase relative to ground. In the singledee construction themissing dee is replaced by a grounded electrode called the dummydee'that mimics the openings of the missing dee. The radiofrequencyenergization in this case is applied between the dee and the groundeddummy dee. The single dee structure need not have an angular extent of180 but may have lesser values provided the proper phase relationship ismaintained between the orbiting particles and the radiofrequency fieldsat the entrance and exit of the dee structure. This requires that theangular distance inside the dee and the angular distance outside the deeeach be equal to an odd multiple of the angle through which the orbitingparticle travels during one-half period of the radiofrequencyenergization. This condition can always be attained if the angulardistance traveled by an orbiting particle during one-half period isequal to 360 divided by an even integer. Examples of dee sizessatisfying the phase requirement are:

180(360/2Xl 90(360/4Xl 135(360/8X3); 108(360/ x3); 150(360/l2X5);126(360/20X7);etc.

The angular velocity (u of an orbiting particle is proportional to themagnetic field strength B in accordance with the equation (2) a =qB/M.Therefore, by proper selection of the radio frequency and the value of Bthe angular distance traveled by the orbiting particle during a halfperiod of the radio frequency can be made equal to 360 divided by aneven integer as required.

A dee structure suitable for use as dee structure 3 in FIG. 1 isillustrated in FIGS. 6, 7, and 8 of the McMillan et al. patent. Any oneof the three identical dee assemblies 89, 96, and 102 may be used. Eachof these assemblies is of the single-dee type, discussed above,comprising, as best seen in FIG. 6, a dee 47 and a grounded dummy dee52. Each of the three dee structures is a complete particle acceleratorin itself operating independently of the other two. Three structuresspaced 120 apart are used to provide three-phase operation giving sixaccelerations per rotation. Single-phase operation with a singledeestructure giving two accelerations per rotation is employed by applicantand could be employed in the patent without affecting the cyclotronoperation other than to reduce the number of accelerations per orbitfrom six to two and, as a result, to proportionately increase the numberof orbits required for a particle to obtain a given velocity. As statedearlier, the dee structure 3 is limited to roughly one-third the angularspace of the cyclotron in order to provide angular space for thegeneration of protons and their extraction from the cyclotron inaccordance with the invention.

In order to simultaneously derive protons at various energy levels fromthe cyclotron of FIG. 1, small stripping foils (either metallic ornonmetallic) are placed in the cyclotron at radial positionscorresponding to the desired energy levels, the energy level increasingwith increasing radius. FIG. 1 illustrates foils positioned at points 4,5, 6, 7, and 8 in the cyclotron for intercepting hydrogen ions at energylevels ranging from to I00 mev. A negative hydrogen ion is a hydrogenatom to which an electron has become attached and consequently consistsof a positive nucleus and two electrons. Such ions can be produced bysubjecting hydrogen gas to an electron rich plasma. The foils strip theelectrons from the negative hydrogen ions leaving the positively chargednuclei or protons which pass through the foil. The number of ionsconverted depends upon the ion current in the cyclotron and the amountof foil area presented to the envelope of the ion current. Except forthe foil at the highest energy location the amount of foil areapresented to the envelope is only a fraction of the envelopecross-sectional area in order to pass sufficient negative ions to feedthe stripping foils at higher energy locations in the cyclotron. Thisarea may be controlled by controlling the extent to which the foil isinserted in the envelope or by turning a narrow ribbon of foil crossingthe ion envelope to control the projected area of the foil exposed tothe circulating ions. F IG. 4 illustrates the first method in which apaddlelike stripping foil 20 lying in a radial plane normal to themedian plane of the cyclotron is inserted into the negative hydrogen ionbeam envelope 2]. FIGS. 5a and 5b are plan and elevation views,respectively, of the second method in which a stripping foil 22 in theform of a ribbon extends across a portion of the negative hydrogen ionbeam envelope 21. The foil may be rotated about an axis 23 normal to themedian plane of the cyclotron, its effective area in this way being madedirectly related to sin 0. With respect to the ion beam envelope 21, aswell known in the art and as described, for example, in Livingood, theorbiting ions in a cyclotron oscillate both radially and axially aboutan equilibrium orbit generating an envelope the size of which dependsupon the amplitude of the oscillations.

Due to the positive charge of the protons, they follow paths of reversedcurvature out of the cyclotron, as illustrated in FIG. 1. The reversalof the direction of curvature results from the fact that the polarity ofthe particle charge changes at the stripping foil from negative for thehydrogen ion to positive for the proton, whereas the direction of themagnetic field remains the same. By proper selection of the azimuthalpositions of the stripping foils, the trajectories of the exitingprotons may be given such directions that if allowed to continue theywould intersect at a single arbitrary point 9 outside the cyclotron. Acombining electromagnet 10, having a circular pole face centered onpoint 9 and a magnet coil 1], is designed to have the proper constantuniform flux and the proper pole face radius, depending upon the protonenergies and exiting directions, to bend the trajectories along a commonpath 12, thus forming a polyergic proton beam. In the embodiment of FIG.1, a second set of stripping foils, generally indicated by referencenumber 13, and a second combining electromagnet 14-15 are provided toproduce a second polyergic proton beam 16 in the same manner as beam 12.As shown, the foils 13 occupy the same radial positions as foils 4-8 sothat beam 16 has the same energy components as beam 12; however,different radial positions could be used if desired to give beam 16 adifferent energy distribution.

The design of the combining magnets will be discussed further withreference to FIGS. 2 and 3. A proton moving through a uniform magneticfield in a plane normal to the flux lines follows a curved path suchthat the produce of the flux density B and the radius of curvature R isa constant the value of which depends upon the kinetic energy of theproton. Tables are available relating the BR product to the kineticenergy of the proton in electron volts. Proton beams that converge on apoint can be turned by a constant uniform magnetic field having acircular edge centered on the point so as to become collinear with aradial line extending from the point provided the flux density, theradius of the circular edge, the particle energies, and the anglebetween any two proton beams, for example, the beams of maximum andminimum energies, are properly related. This is illustrated in FIG. 2where (as seen also in FIG. 1) 9 is the point on which the lowest energyproton beam 17 and the highest energy proton beam 18 converge, 10 is thecircular pole face of the combining magnet centered on point 9, R is theradius of the pole face or circulat edge of the field, R is the radiusof curvature of the lowenergy beam. R is the radius of curvature of thehigh-energy beam, 0, is the low-energy beam turning angle, 6 is thehighenergy beam turning angle, and A is the angular separation of thehighand low-energy beams. From the geometry of FIG. 2 the followingrelationships are seen to exist:

tan

6) x 2(tan" Therefore, for a given value of A and given maximum andminimum proton kinetic energies which determine the values of theconstant (BR) and (BR) the correct value of the product B R is thatwhich satisfies equation (6). Either B, or

R may be chosen and the other determined by dividing the chosen valueinto the value of the product. HO. 3 relates graphically the B R productto the values of 9, A, and proton energies.

In lieu of using a BR product table the values of R and R may becomputed for a chosen value of B as follows:

where K, and K are the kinetic ehergies in electron volts of the lowestand highest energy protons (2O mev. and lOO mev. in the example given),m is the mass of a proton, and q is the proton charge (approx. l.6X l0coulombs). Substituting the computed values of R and R together with thegiven value of )t in equation (3), the correct value of R is that whichsatisfies this equation.

lclaim:

1. Apparatus for producing a polyergic beam of protons comprising: anegative hydrogen ion cyclotron having a plurality of stripping foilspositioned at different radial positions for converting negativehydrogen ions at different energies into protons at different energieswhich follow trajectories of reversed curvature out of the cyclotron,said foils having such azimuthal locations that the trajectories of theexiting protons converge toward a point outside said cyclotron; andmeans at said point for bending said converging trajectories into acommon polyergic beam.

2. Apparatus as claimed in claim 1 in which said bending means comprisesmeans producing a uniform constant magnetic field having a circular edgecentered on said point, the product of the flux density of said fieldand the radius of said circular edge having such value, depending uponthe angular separation of any two of said converging proton trajectoriesand the energies of the protons therein, that all of said convergingtrajectories are turned into collinearity with a radial line extendingfrom said point.

3. Apparatus as claimed in claim 1 in which the angular space of thecyclotron has three sectors with the particle-accelerating electrodestructure of the cyclotron in one of the sectors and said strippingfoils in another; and a second set of stripping foils in the remainingsector having different radial and azimuthal positions as required toproduce protons at various energies which exit the cyclotron alongtrajectories that converge on a second point outside the cyclotron inthe same manner as the first-named foils, and means at said second pointfor bending the converging trajectories into a second common polyergicbeam.

1. Apparatus for producing a polyergic beam of protons comprising: anegative hydrogen ion cyclotron having a plurality of stripping foilspositioned at different radial positions for converting negativehydrogen ions at different energies into protons at different energieswhich follow trajectories of reversed curvature out of the cyclotron,said foils having such azimuthal locations that the trajectories of theexiting protons converge toward a point outside said cyclotron; andmeans at said point for bending said converging trajectories into acommon polyergic beam.
 2. Apparatus as claimed in claim 1 in which saidbending means comprises means producing a uniform constant magneticfield having a circular edge centered on said point, the product of theflux density of said field and the radius of said circular edge havingsuch value, depending upon the angular separation of any two of saidconverging proton trajectories and the energies of the protons therein,that all of said converging trajectories are turned into collinearitywith a radial line extending from said point.
 3. Apparatus as claimed inclaim 1 in which the angular space of the cyclotron has three sectorswith the particle-accelerating electrode structure of the cyclotron inone of the sectors and said stripping foils in another; and a second setof stripping foils in the remaining sector having different radial andazimuthal positions as required to produce protons at various energieswhich exit the cyclotron along trajectories that converge on a secondpoint outside the cyclotron in the same manner as the first-named foils,and means at said second point for bending the converging trajectoriesinto a second common polyergic beam.